Documentation change (#3672)

Co-authored-by: Florian Albrechtskirchinger <falbrechtskirchinger@gmail.com>

docs/examples/nlohmann_json_namespace_begin.c++17.cpp

@@ -0,0 +1,33 @@
+#include <iostream>
+#include <optional>
+#include <nlohmann/json.hpp>
+
+// partial specialization (see https://json.nlohmann.me/features/arbitrary_types/)
+NLOHMANN_JSON_NAMESPACE_BEGIN
+template <typename T>
+struct adl_serializer<std::optional<T>>
+{
+    static void to_json(json& j, const std::optional<T>& opt)
+    {
+        if (opt == std::nullopt)
+        {
+            j = nullptr;
+        }
+        else
+        {
+            j = *opt;
+        }
+    }
+};
+NLOHMANN_JSON_NAMESPACE_END
+
+int main()
+{
+    std::optional<int> o1 = 1;
+    std::optional<int> o2 = std::nullopt;
+
+    NLOHMANN_JSON_NAMESPACE::json j;
+    j.push_back(o1);
+    j.push_back(o2);
+    std::cout << j << std::endl;
+}